Human papillomaviruses (HPVs) are the major cause of anogenital dysplasias and carcinomas in both men and women. The HPV E6 and E7 proteins underlie the ability of papillomaviruses to cause disease. We have used short interfering double strand RNAs directed against HPV-18 E6 and E7 (RNA interference) to successfully cause growth arrest and senescence in HeLa cells. Similar results have been obtained in SW756 cells (a second HPV-18-transformed cell line), but not in C33A cells (a non-HPV-transformed cervical carcinoma cell line). Our data indicate that anti-E6/E7 RNA interference can reduce E6 and E7 expression, and that the reduction of E6 and E7 expression could be used to treat both pre-malignant HPV infections and HPV-related cancers. A barrier to the therapeutic use of RNA interference is the inefficiency with which double strand phosphodiester RNAs enter cells. To promote the entry of therapeutic interfering RNAs into cells without the use of lipofection reagents, we have begun to evaluate boranophosphate- and phosphorothioate-backbone RNAs for their abilities to enter cells and to induce RNA interference. Our data indicate that both boranophosphate- and phosphorothioate-backbone RNAs can be used to induce RNA interference of green fluorescent protein expression in HeLa cells. Boranophosphate-modified single-stranded siRNAs are also active. We predict that boranophosphate and phosphorothioate RNAs will enter cells efficiently, and will induce RNA interference with E6 and E7 expression, resulting in growth arrest and senescence of HeLa cells in culture. RNA interference holds significant early promise for the treatment of many disorders, including HPV infections and HPV-related malignancies. This work is an initial step in the evaluation of boranophosphateand phosphorothioate-based RNA interference as a therapeutic modality.